Section 9-3. GMRP

9-3. GMRP

• GMRP is an industry standard multicast flooding control protocol defined in IEEE 802.1p.

• A host uses both IGMP (Layer 3) and GMRP (Layer 2) to coordinate its multicast activities. When it wants to join a multicast group, it sends an IGMP join request along with a GMRP join request.

• The switch forwards IGMP control packets on to the multicast router. GMRP traffic is used by the switch to determine which switch ports to add to a multicast group.

• The switch periodically queries hosts with a GMRP leave-all message. Hosts that want to continue participating in a multicast group must respond with a join request. If not, they can either send a GMRP leave message or just not respond at all.

Configuration

1.

Enable GMRP.

a. Enable GMRP on all VLANs and switch ports:

COS	set gmrp {enable | disable}
IOS	N/A

b. Enable or disable GMRP on specific ports:

COS	set port gmrp mod/ports... {enable | disable}
IOS	N/A

After GMRP has been enabled on the switch, it is enabled on all VLANs and switch ports. You can disable it on ports where it is not needed.

2.

Identify ports where routers are connected:

COS	set gmrp fwdall enable mod/port...
IOS	N/A

The switch must know where any multicast routers are connected so that it can forward all multicast traffic there. GMRP traffic is not forwarded because it is only used by hosts and switches to constrain multicast traffic.

3.

Specify the GMRP registration type:

COS	[View full width] set gmrp registration {normal | fixed | forbidden} mod/port...
IOS	N/A

The registration type controls how a switch port will participate in registering hosts to multicast groups. In normal mode (the default), hosts are free to dynamically register and leave multicast groups on the port. In fixed mode, the current multicast registrations are frozen; no further joins or leaves are allowed. In forbidden mode, all multicast registrations on the port are released and no further joins are allowed.

4.

(Optional) Tune the GARP/GMRP timers.

TIP

The GARP timers are used to define when control messages, such as join and leave requests, can be sent or accepted. If you decide to adjust these timers, be sure to adjust them consistently on all switches and host devices.

a. (Optional) Adjust the join timer:

COS	set gmrp timer join timer-value
IOS	N/A

The join timer is used to pace GARP control message transmissions (join requests, for example). These messages can only be sent every timer-value (1 to 2,147,483,647 milliseconds, default 200 ms).

b. (Optional) Adjust the leave timer:

COS	set gmrp timer leave timer-value
IOS	N/A

When a host sends a leave request, the switch port could potentially be removed from a multicast group. The switch waits for a leave time to listen for any join requests on the port before removing the registration. The leave timer is set to timer-value (1 to 2,147,483,647 milliseconds, default 600 ms). The leave timer must be at least three times greater than the join timer.

c. (Optional) Adjust the leave-all timer:

COS	set gmrp timer leaveall timer-value
IOS	N/A

If the switch doesn't receive a response from a registered host within timer-value (1 to 2,147,483,647 milliseconds, default 10,000 ms or 10 seconds), the host is removed from any multicast groups. The leave-all timer must be greater than the leave timer.

Displaying Information About GMRP

Table 9-4 lists some switch commands that you can use to display helpful information about GMRP.

Table 9-4. Switch Commands to Display GMRP Information

Display Function	Switch OS	Command
GMRP status	COS	show gmrp configuration
	IOS	N/A
GMRP statistics	COS	show gmrp statistics [vlan]
	IOS	N/A
GARP timers	COS	show garp timer
	IOS	N/A